1. Field of the Invention
The present invention relates to an optical transmission device provided with light guides each having light incidence/emission sections and a substrate over which optical elements are arranged matching the light incidence/emission sections.
2. Description of the Prior Art
Known techniques in this regard include ones by which, when fixing light guides to a substrate, optical components are fabricated with the light guides and the substrate being matched in the coefficient of linear expansion. For instance, the Japanese Published Unexamined Patent Application No. 2001-4850 discloses a technique by which the coefficient of linear expansion of the substrate is matched to the material of optical waveguide. The yield of component production and the reliability of produced components are expected to be enhanced by the use of such a technique.
There are various available methods for fixing the light guides and the substrate including, for instance, one by which a positioning section provided on each light guide is butted against the reference plane of the substrate and another by which light guides are embedded in the substrate. Such light guides and a substrate can be formed by cutting and grinding, or injection-molding, plastic materials. For instance, in a light guide made up of a light transmitting section and a light incidence/emission section, where the distance (in the lengthwise direction) from a reference plane provided in the light transmitting section to an incidence/emission section (central part) at the tip is L mm and the coefficient of linear expansion is M×10−5/° C. for the light guides and N×10−5/° C. for the substrate, a temperature change by T° C. would result in a difference in length by L×(M−N)×10−5×T (mm) from the reference plane to the incidence/emission section at the tip. This difference in length invites a positional lag between each light incidence/emission section of each light guide and each light incidence/emission optical element arranged on the substrate to match the light incidence/emission section. This positional lag may give rise to a significant loss in optical coupling between the light incidence/emission sections of each light guide and the light incidence/emission optical elements. In order to prevent such a positional lag, the materials are so selected as to match the light guides and the substrate in the coefficient of linear expansion.
However, even if materials substantially equal in the coefficient of linear expansion are selected for the light guides and the substrate, a significant loss in optical coupling between the light incidence/emission section of each light guide and the light incidence/emission optical element sometimes occurs. Such a loss in optical coupling has to be avoided because it would bring down the overall light utilizing efficiency of the device.